Mechanism of electrophilic substitution at a saturated carbon atom. Part XI. Bimolecular and unimolecular substitution of mercury for gold in alkylgold complexes

Abstract

Reactions in which gold, bound to methyl, ethyl, and 1-cyano-1-ethoxycarbonylpentyl groups in their triphenylphosphinegold(I) derivatives, is displaced therefrom by entering mercury from mercuric or alkylmercuric halides, acetates, or nitrates have been examined kinetically in dioxan, acetone, dimethylformamide, and dimethyl sulphoxide solvents. Most of the mercury-for-gold substitutions are second-order processes, and, where this is so, the effect on rate of changes of the anion in the mercurial, and also the kinetic effect of added salts, make clear that the bimolecular electrophilic mechanism of substitution, S_E2, is under observation. However, the reactions in which hold from the cyano(ethoxycarbonyl)pentylgold complex is replaced by mercury in solvents containing dimethyl sulphoxide are kinetically of first order in the gold complex, and zeroth order in the mercurial. They illustrate the unimolecular electrophilic mechanism, S_E1. In this mechanism all mercurials react at the same rate, which is the rate of formation of a carbanion by heterolysis of the carbon–gold bond. The reaction between mercuric bromide and one gold(III) complex, viz. trimethyl(triphenylphosphine)gold(III), has been examined. In dioxan and acetone as solvents, the gold is displaced from one methyl group only, to be replaced by mercury in an S_E2 process.